The invention relates to a printing form for a rotary relief-printing method, in which the printing dots transferring the printing ink constitute a half-tone screen and are superimposed by a relief structure constituted by indentations.
In the rotary relief-printing method, the ink is transferred onto the print directly by means of the printing form. This form is manufactured as a printing plate and clamped onto the jacket of a printing block cylinder. When a printing motif or a printing image is being printed, it is formed as a generally regular printing screen so as to generate so-called half-tones, whose raised printing dots constitute plane printing surfaces which are inked by means of a suitable device and transfer the ink onto the material to be printed.
A printing form of the type described in the opening paragraph is known from DE-C1-43 24 577. The fine screen, which in this printing form is constituted as a relief structure in the form of indentations and is superimposed on the half-tone screen, allows a homogeneous and adequate transfer of ink onto a non-absorptive material, for example glass. Both the half-tone screen and also the fine screen are unambiguously defined by way of the so-called screen number, i.e. the number of printing lines or printing dots per centimeter, calculated in the direction of the shortest distance and the percentage of surface coverage. Since the individual printing surfaces or printing dots are arranged in lines, it is common practice to speak of lines per centimeter. The screen number thus represents the division or dot spacing. The dot size is determined by the percentage of surface coverage. The percentage of surface coverage is the ratio between the surfaces of the raised printing dots and the total surface of the mosaic printing motif. The printing dots do not need to be circular but may fundamentally have any arbitrary geometrical shape.
In the known printing form, the outer boundary of the printing dots of the half-tone screen may be interrupted by the structure of the fine screen so that a well-defined boundary of the printing dots is no longer guaranteed. This is caused by the fact that only the screen number and the percentage of surface coverage is taken into account when selecting the fine screen. However, the areas in which the fine screen is superimposed on the half-tone screen are not taken into account. Consequently, fine screen dots (indentations) may be only partially superimposed on the printing dots in their border areas so that the border area of such a printing dot is interrupted. This effect is particularly essential in those cases in which only a small number of fine screen dots having a relatively large surface is allocated to a printing dot of the half-tone screen. As a result, an irregular boundary, or serrated edges, can clearly be recognized in the printed image of such printing dots. This has a negative effect, particularly when manufacturing flat-panel display screens, in which the flexographic printing method may be used for providing the glass plate with a luminescent material coating.